1. Field of the Invention
This invention generally relates to a high-density cement composition for preventing gas migration in deep gas wells.
2. Background of the Invention
Gas migration through cement columns has been an industry problem for many years. The most problematic areas for gas migrations are in deep gas wells. For instance, approximately 80% of wells in the Gulf of Mexico have gas transmitted to surface through cemented casings. In Saudi Arabia, the most problematic operations for gas migration are those involving deep gas wells. In such instances, drilling fluid densities as high as 163 pcf (pounds per cubic foot) are used to control gas or formation fluid influx. To control gas migration, cement densities for successfully cementing of the zone of interest can be as high as 170 pcf. As a cement slurry sets, hydrostatic pressure is reduced on the formation. During this transition, reservoir gases can travel up through the cement column resulting in gas being present at the surface. The permeable channels from which the gas flows cause operational and safety problems at the well site.
Causes of gas channeling include: 1) bad mud/spacer/cement design that allows passage of water and gas, resulting in failures in cementing operations, 2) high fluid loss from cement slurries, which causes water accumulation and results in micro-fractures within the cement body, and 3) cements not providing sufficient hydrostatic pressure to control the high pressure formation.
Good displacement practices with the use of stable, fast-setting, low-fluid-loss slurries are important in solving gas zonal isolation problems in many, but not all, cementing operations failures. The resulting slurry properties are affected by the slurry composition and the well conditions. The slurry composition effects include the dehydration of the liquid phase, gelation of the slurry, settling of the solid particles, and packing of the solid particles. The setting of cement starts when water is first in contact with the cement. Initially, the cement slurry column behaves as a pure fluid and fully transmits the hydrostatic pressure. As the cement starts to set, settling and packing of the slurry continues. Once the cement structure starts to gel, the pore pressure inside the cement column starts to decrease until it becomes equal to the pressure of the formation. As the cement pore pressure decreases, this allows the gas to invade the cement pore spaces. If the cement permeability to gas is high and gas invasion occurs, the gas can permeate throughout the cement matrix, charging it with enough gas (and pore pressure) to inhibit the hydration process from closing the pore spaces. When the gas pressure is higher than the hydrostatic pressure after the cement initially sets, a channel forms and gas continues to migrate even after decreasing the formation pressure.
There is a strong relationship between water separation in cement slurry and the loss of hydrostatic head of the cement columns. One way to improve gas migration control is by using fluid loss and expansion additives. Fluid loss additives retain the water needed for hydration of cement and slowly release it during the complete hydration process. In addition, fluid loss additives minimize the ability of fluids to flow though the cement porosity. Using expansion additives can improve bonding at the casing/cement and cement/formation interfaces.
Gas can migrate when the cement is in the slurry form, if densities are not well designed. Slurry setting will prevent hydrostatic pressure transmission, and consequently, will reduce pressure facing the gas zone. Slurries that minimize this transition time are desirable. Hardened cement should be resistant to mechanical and thermal stresses to avoid fractures, which would become an easy path for the gas. Optimizing slurry design includes designing compositions to have no free water and to minimize fluid loss. Adjusting cement properties based on conventional testing is not enough to confirm that the slurry will be gas migration resistant. Testing slurries on a gas flow simulator is a useful tool for the optimizing process.
The use of latex additives can help control gas migration in cement because cement pore pressure drop is delayed and the transition time between the liquid and set state is shortened. However, as long as the cement behaves as a true liquid, gas can channel up in the annulus when gas pressure is higher than cement hydrostatic pressure. Thus, density of the cement must be designed according to the formation pressure and the fracture gradient, and must be controlled during the entire cementing operation. Latex additives can assist in the prevention of gas/fluid migration during the setting of cement. For wells that have considerable fluid or gas flow, latex may be recommended. On wells with mud weights equal to or greater than 135 pcf, latex may be recommended. For wells with drilling fluid densities that are less than 120 pcf, conventional dry fluid loss additives may also be recommended. These wells with high mud density usually have had considerable flow from the formation. The time needed to build the mud volumes to obtain the proper mud weight is usually more than a day.
Expanding cement additives are useful for wells that will be drilled with mud densities that are less than 15 pcf from the previous hole section. The reduction of pressure from reducing the mud density can cause the casing to shrink. This shrinkage can cause the cement-casing bond to break, which will allow for gas flow. This situation is more likely to occur as the depth increases. Expanding additives are also generally recommended for cement jobs where a gas producing formation is being cemented and the depth is greater than 10,000 ft.
Cementing an unbalanced wellbore in high-pressure formations is challenging because the cement will migrate up in the cement column. One preferred procedure is to kill the well using mud and then perform the cementing operation to reduce the gas migration potential. In Saudi Arabia, the wells are generally killed before cementing operations, however, higher-pressure formations with higher gas migration problems require higher cement density.
The most common problem associated with heavy weight cement slurries using hematite is settling. Sometimes, settling can be controlled by anti-settling chemicals in the lab. However, controlling hematite settling in the field has historically proven difficult.
Portland cement has tricalcium silicate (C3S) and dicalcium silicate (C2S). When mixed with water, both hydrate to form calcium silicate hydrate (C—S—H) gel. The C—S—H gel can provide good compressive strength for the cement at temperature up to 230° F. However, at higher temperatures, C—S—H gel forms a phase called alpha dicalcium silicate hydrate (α-C2SH) which decreases the compressive strength and permeability of set cement. To prevent the formation of α-C2SH, the lime-silica ratio (C/S) is reduced by addition of silica-based materials. The addition of silica material to cement, when hydrated, will form a phase known as tobermorite (C5S6H) at 230° F. instead of α-C2SH phase and high strength cement results.
Despite these various approaches to cement compositions, current high-density cement formulations do not provide good gas migration prevention due to settling and increase in permeability. To solve the settling problem and reduce permeability, a new formula is needed to prevent gas migration problems in cementing high-pressure formations.